1. A catecholamine β‐adrenoreceptor agonist, hexoprenaline, was examined in vitro on five guinea‐pig tissues and its potency relative to isoprenaline (as 100) obtained. 2. Hexoprenaline clearly delineated between those tissues classified as containing β2‐adrenoreceptors (trachea, hind limb blood vessels and uterus; relative potencies 219, 110 and 76 respectively) and those classified as containing β1‐adrenoreceptors (atria and ileum; relative potencies 3·3 and 1·0 respectively). 3. Hexoprenaline differed from some previously studied noncatecholamine β‐adrenoreceptor agonists in being only two‐fold less potent, relative to isoprenaline, as a vasodilator in perfused hind limb than as a tracheal relaxant. Copyright